Abstract

This paper examines the generation of a particular noise, referred to as crackle, first observed by Ffowcs Williams et al. in a full-scale jet engine. Schlieren pictures along with far-field acoustic data obtained for high-temperature axisymmetric supersonic jets were used to examine the crackle characteristics. Experimental data from a full-scale engine were also used to validate the features obtained from small-scale laboratory experiments. The experimental data cover a velocity range from 600 m/s to 1.05 km/s. A schlieren picture of the radiation field of an M = 2 high temperature jet (U(j) = 1.05 km/s), depicts shock-wavelike structures emanating from the jet and propagating in the downstream direction at shallow angles to the jet axis. The frequency of occurrence of these intense waves increases with jet exit velocity/temperature. It is suggested that microexplosions of the cold entrained ambient fluid in the hot jet generate these intense waves, which are believed to be associated with crackle. When an entrained cold lump of fluid is exposed suddenly to the hot jet fiuid, it expands, giving rise to a monopole source of sound. The convection of the monopoles at supersonic speed leads to the observed directionality of the wave front/crackle sound. (Author)